This invention concerns new electrocatalysts active for the electroreduction of oxygen, and particularly a new polymeric carbon catalyst.
Within the field of electrochemistry, there is a well-known type of an electrolytic cell known as a chlor-alkali cell. Basically this is a cell wherein chlorine gas and caustic soda are produced by passing an electric current through a concentrated brine solution containing sodium chloride and water. A large portion of the chlorine and caustic soda for the chemical and plastics industries is produced in chlor-alkali cells. The cathodes employed in such chlor-alkali cells are subjected to the corrosive environment of the caustic soda.
Chlor-alkali cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available NAFION.RTM. manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet, as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a noble metal coating to yield what is known in the art as a dimensionally stable anode.
One of the unwanted by-products present in a chlor-alkali cell is hydrogen which forms at the cell cathode according to the following reaction 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.-, E.degree.=-0.828 volts. This production of hydrogen increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
Fairly recently, attention has been directed in chlor-alkali cell technology to various forms of oxygen cathodes. In operation, an oxygen electrode is positioned in a chlor-alkali cell and contacted with an aqueous electrolyte on one side and an oxygen-containing gas on an opposing side. Oxygen electrodes are porous and allow the electrolyte and the oxygen-containing gas to permeate into the electrode to form a three-phase interface between the electrolyte, the oxygen-containing gas and the electrode surface. Electrical energy is supplied through the cathode and causes reactions to occur between the electrolyte and the oxygen. The reactions are throught to occur as a two-step reaction, with the overall reaction described by: ##EQU1##
This represents a potential theoretical voltage savings of 1.229 volts. If only a portion of this theoretical voltage savings could be realized, a substantial amount of energy could be saved.
Catalysts active for the elctroreduction of oxygen are commonly placed on the electrode surface to enhance the reactions. Commonly used catalysts include, for example, gold, osmium, palladium, platinum, silver, and carbon. Carbon is the preferred catalyst because it is substantially cheaper than most other catalysts and is more readily available. However, carbon is catalytically active for only the first step of the two-step reaction. Thus, when carbon is used as the catalyst, another catalyst (active for the second step of the two-step reaction) is needed.
A chemically stable, electrically conductive carbon catalyst active for both steps of the two-step oxygen reduction reaction would be highly desirable. It is an object of the invention to provide such a catalyst.